Saturation magnetostriction and its lowtemperature variation inferred for natural titanomaghemites; implications for internal stress control of coercivity in oceanic basalts
Hodych, J. P., and J. Matzka (2004),
Saturation magnetostriction and its lowtemperature variation inferred for natural titanomaghemites; implications for internal stress control of coercivity in oceanic basalts,
Geophysical Journal International, 157(3), 10171026, doi:10.1111/j.1365246X.2004.02231.x.
 Abstract
 Highly oxidized titanomaghemite in oceanic basalts often carries remanent magnetization of high coercivity (stability), helping preserve the oceanic magnetic anomaly pattern. We study the source of this high coercivity in four oceanic basalts (from ODP sites 238, 572D, 470A and 556) containing highly oxidized titanomaghemite (titanium content parameter x approximate to 0.55 and oxidation parameter z approximate to 0.9 on average). Most of the titanomaghemite is likely in singledomain grains with uniaxial anisotropy because the ratio of saturation remanence J(RS) to saturation magnetization J(S) approaches 0.50 (J(RS)/J(S) = 0.46 on average). We show that the uniaxial anisotropy is very likely magnetostrictively controlled through internal stresses sigma (i) in the titanomaghemite grains. This allows us to use a novel indirect method to estimate the saturation magnetostriction lambda(S) of the titanomaghemite. A saturation remanence J(RS) is given along the axis of a cylindrical sample of each basalt. Then a small compression sigma is applied repeatedly along this axis and the reversible change DeltaJ(Rs) in J(Rs) is measured. Combining equations from singledomain theory for this piezomagnetic effect and for the sample's coercive force HC gives lambda(S) = 1.39H(C)J(S) 1/sigma DeltaJ(RS)/J(RS) (using cgs units, or with HC in mT, J(S) in kA/m and sigma in Pa). This yields fours lambda(S) estimates (with ca 50 per cent expected error) ranging from 3 x 10(6) to 10 x 10(6) and averaging 6 x 10(6). Theory for the piezomagnetic effect yields four sigma(i) estimates averaging 2 x 108 Pa. This is similar to the internal stress magnitude thought to be responsible for the high coercivity of ballmilled singledomain titanomagnetite (x approximate to 0.6) and natural singledomain haematite. We also show that cooling to 120 degreesK causes H(C)J(S) for each oceanic basalt to vary in approximate proportion to (1T/TC)(n) with n between 1.9 and 2.0 (where T is temperature and TC is Curie point, both in degreesK). This implies that lambda(S) of titanomaghemite with x approximate to 0.55 and z approximate to 0.9 also varies in approximate proportion to (1T/TC)(n) with n near 1.9 or 2.0 on cooling to 120 degreesK (assuming that sigma(i) remains constant on cooling). Our results support the hypothesis that coercivity (magnetic stability) is often magnetostrictively controlled by internal stresses in the highly oxidized titanomaghemites typical of oceanic basalts older than ca 10 Myr. We suggest that this hypothesis can be further tested by more extensive observation of whether cooling to 120 degreesK often causes H(C)J(S) of such basalts to vary in approximate proportion to (1T/TC)(n) with n near 1.9 or 2.0.
 BibTeX

@article{id150,
author = {J. P. Hodych and J. Matzka},
journal = {Geophysical Journal International},
number = {3},
pages = {10171026},
title = {{Saturation magnetostriction and its lowtemperature variation inferred for natural titanomaghemites; implications for internal stress control of coercivity in oceanic basalts}},
volume = {157},
year = {2004},
language = {en},
doi = {10.1111/j.1365246X.2004.02231.x},
}
 EndNote

%0 Journal Article
%A Hodych, J. P.
%A Matzka, J.
%D 2004
%N 3
%V 157
%J Geophysical Journal International
%P 10171026
%T Saturation magnetostriction and its lowtemperature variation inferred for natural titanomaghemites; implications for internal stress control of coercivity in oceanic basalts